For an electronic commutation of a drive motor, the control unit controlling the commutation must have rotor position information in order to be able to cause a respective inversion of the direction of current flow to the stator coils at the correct time. One or a plurality of sensors may be provided for the detection of the rotor position information, such as Hall sensors responsive to magnetic fields, which detect the passing magnetic field of the permanent magnetic motor rotor or of a separate sensor magnet.
It is preferred for cost reasons not to provide a separate sensor, if possible. During forced pauses of current supply to the motor, a so-called sensorless rotor position detection may be provided by evaluating the induction voltage, the so-called EMF, induced in the stator coils by the permanent magnetic rotor, and to conclude the rotor position based on the time sequence of the induction voltage. However, this method requires a substantial computational capacity of the control unit. This method is further only suitable with small inductivities, since large inductivities require a very long pause in current supply in order to observe an interference-free EMF induction voltage. In particular with small electric drive motors for smaller motor vehicle auxiliary units, such as, for example, water circulating pumps, high inductivities are also common, for example, for physical reasons, so that this method is not suited for small drive motors.